The present invention relates to the efficient production of oxygen from the electrolysis of water, and particularly to an improved catalytic anode for electrochemical oxygen evolution from alkaline water, and the method of making the same.
An electrochemical cell is a device which has as basic components at least one anode and one cathode and an electrolyte. The cell may use electrical energy to achieve a chemical reaction, such as the oxidation or reduction of a chemical compound as in an electrolytic cell. Alternatively, it can convert inherent chemical energy in a conventional fuel into low voltage direct current electrical energy as in a fuel cell. The activity of such an electrochemical device is usually expressed in terms of exchange current density or overpotential. The exchange current density (.sup.i o) is the current density of the reversible potential (e.g., 0.4 V vs. NHE at 25.degree. C. in 1M NaOH for oxygen reaction) and the overpotential measures how much the potential has departed from the reversible value to drive a reaction of a given current density.
Since the reaction rate in an electrochemical device is given by a current density, high electrocatalytic activity is associated with a high value of current density and a low value of overpotential. If the electrodes in such a cell are of common electrode material, such as e.g. iron or nickel, they tend to have low activity. The activity can be improved by coating such electrodes with precious metal electrocatalysts, such as e.g. platinum, iridium or ruthenium. The level of precious metal required for high activity and stability generally leads to high costs.
The above problems are particularly acute in electrochemical cells used, for example, for the electrolysis of water to produce hydrogen and oxygen. Hydrogen is a versatile raw material. It is, for example, a most desirable source of fuel and energy due to the clean and non-toxic nature of its combustion products. In addition, it is used, for example, in the fertilizer, metallurgical and petrochemical industries. While demand for hydrogen is increasing, production costs from conventional sources are also increasing. Water is a natural resource which is readily and abundantly available, and from which hydrogen and oxygen can be produced by electrolysis. However, the cost of the electrocatalysts hitherto available has detracted from the commercial viability of water electrolysis technology.
Similarly, the oxygen produced by water electrolysis has wide utility as a chemical feedstock. If produced very cheaply or, as an essentially free by-product of hydrogen production, oxygen could also be used as a high-temperature combustion promoter which could make usable as fuels a wide range of poorly combustible materials.
The rapid increase in costs of conventional fuels, particularly hydrocarbons, in recent years, as well as increasing concern over the environmental effect of utilizing hydrocarbon fuels, has prompted increased research and development efforts in the area of improving catalysts, methods and apparata for electrochemical water splitting to produce pure hydrogen and oxygen. Most current water electrolyzers utilize an approximately 25-30% alkaline water solution as the electrolyte. The principal cause of loss of efficiency in such alkaline water electrolysis is the irreversibility of the oxygen evolution reaction on the metals and alloys used as anodic materials, including specifically the growth of poorly conducting oxide films on the anodes.
Among the more commonly used anode materials for such cells are nickel and platinum. Nickel, while reasonable in terms of cost, shows both a low current density, with resultant poor oxygen evolution activity, and a tendency to form insulating oxide films. Platinum, used pure as an anode material, or as a catalytic coating on nickel or other metal or alloy anodes, increases current density and oxygen evolution activity of the cell, but at very substantially increased costs, which make the process economically unattractive.
One of the most active anode materials presently known for alkaline water electrolysis is the spinel-type material, NiCo.sub.2 O.sub.4, which has a reported activity many times higher than platinum. Nevertheless, such material has not proved attractive for large scale use because some of the oxides on the surface tend to drop off on prolonged testing at high current densities. For discussions of spinel-type NiCo.sub.2 O4 as an anode material see, for example, A. C. C. Tseung and S. Jasem, Electrochem. Acta., 22, 31 (1977); A. C. C. Tseung, S. Jasem, and M. N. Mahmood, Hydrogen Energy System [T. V. Vegiroglu and W. Seifritz, Editors, Vol. 1, p. 215, Pergamon, Oxford (1978)]; and S. Trasatti and G. Lodi, Electrodes of Conductive Metallic Oxides, [S. Trasatti, Editor, p. 521, Elsevier, N.Y. (1981)].
It is accordingly the principal object of the present invention to provide an improved anode material for oxygen production by electrolysis.
A further object is to provide an anode material which exhibits improved catalytic activity for oxygen evolution in alkaline water electrolysis, and improved stability as compared to prior art electrodes.
Another object is to provide an improved process for the manufacture of such material and for the production of electrodes from such material.